I am not sure I am able to be much help but
You will remain in terminal 3 at IGI airport in Delhi and there is an international transfer section.
I would have thought you will not have to reclaim your baggage - you do if transferring to a domestic flight which is the only experience I have
I doubt you will need an Indian visa just what is required for Sri Lanka.
Air India ought to be able to tell you as they will either not let you travel or get fined if you do if you do not have the right documentation.
Otherwise you could try one of the companies that assist with visas e.g. CIBT
CIBTvisas
They might charge you a fee for their advice.